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SSL_CTX_SET_CERT_VERIFY_CALLBACK(3) Library Functions Manual SSL_CTX_SET_CERT_VERIFY_CALLBACK(3)

SSL_CTX_set_cert_verify_callback
set peer certificate verification procedure

#include <openssl/ssl.h>
void
SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx, int (*callback)(X509_STORE_CTX *, void *), void *arg);

SSL_CTX_set_cert_verify_callback() sets the verification callback function for ctx. SSL objects that are created from ctx inherit the setting valid at the time when SSL_new(3) is called.

Whenever a certificate is verified during a SSL/TLS handshake, a verification function is called. If the application does not explicitly specify a verification callback function, the built-in verification function is used. If a verification callback callback is specified via SSL_CTX_set_cert_verify_callback(), the supplied callback function is called instead. By setting callback to NULL, the default behaviour is restored.
When the verification must be performed, callback will be called with the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg). The argument arg is specified by the application when setting callback.
callback should return 1 to indicate verification success and 0 to indicate verification failure. If SSL_VERIFY_PEER is set and callback returns 0, the handshake will fail. As the verification procedure may allow the connection to continue in case of failure (by always returning 1) the verification result must be set in any case using the error member of x509_store_ctx so that the calling application will be informed about the detailed result of the verification procedure!
Within x509_store_ctx, callback has access to the verify_callback function set using SSL_CTX_set_verify(3).

ssl(3), SSL_CTX_load_verify_locations(3), SSL_CTX_set_verify(3), SSL_get_verify_result(3)

SSL_CTX_set_cert_verify_callback() first appeared in SSLeay 0.6.1 and has been available since OpenBSD 2.4.
Previous to OpenSSL 0.9.7, the arg argument to SSL_CTX_set_cert_verify_callback() was ignored, and callback was called simply as int (*callback)(X509_STORE_CTX *). To compile software written for previous versions of OpenSSL, a dummy argument will have to be added to callback.

Do not mix the verification callback described in this function with the verify_callback function called during the verification process. The latter is set using the SSL_CTX_set_verify(3) family of functions.
Providing a complete verification procedure including certificate purpose settings, etc., is a complex task. The built-in procedure is quite powerful and in most cases it should be sufficient to modify its behaviour using the verify_callback function.

SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.
March 27, 2018 OpenBSD-current